They are usually movies that don't get funded to go on DVD by major studios or something.

Criterion is a company that does that and they print very short runs of the DVD and include a shit ton of Extras.

Not true. Criterion focuses on what they consider important and/or groundbreaking movies, or what they say are "the defining moments of cinema" or something like that. They do amazing transfers and generally have incredible extra features.

That said, they have a love affair with Wes Anderson (seriously, WTF?), and occasionally come out with strange releases (Armageddon? The Rock? Seriously, WTF?).